1. Field of the Invention
The present invention relates to a superconducting magnet apparatus combined with a cryostat cooled by a refrigerator, and a maintenance method of the refrigerator used for the superconducting magnet apparatus. The present invention relates particularly to a superconducting magnet apparatus suited for a single crystal pulling device and a maintenance method for a refrigerator used for the superconducting magnet apparatus.
2. Description of the Related Art
Recently, a Gifford-McMahon (“GM”) refrigerator R, as shown in FIG. 1, is being used as a cooling device for a cryostat (cryogenic vacuum vessel or chamber) in replace of liquid helium. The GM refrigerator R primarily includes a motor drive M, a plurality of stages (two in this example) of cooling cylinders C1, C2, and displacers D1, D2 that include heat reservoirs and are driven by the motor drive M to reciprocate in the cylinders C1, C2.
A first-stage cold head H1 is provided at the lower end of the first-stage cooling cylinder C1, and a second-stage cold head H2 is provided at the lower end of the second-stage cooling cylinder C2. An upper opening rim portion of the first-stage cooling cylinder C1 has a flange 4 for mounting the motor drive M and for installation to a vacuum vessel or chamber, which will be discussed hereinafter. The displacers D1, D2 are inserted into the first and second-stage cooling cylinders C1, C2 through an opening in the flange 4.
The GM refrigerator R having the first and second-stage cold heads H1, H2 enables the first-stage cold head H1 to be set to cryogenic levels ranging from 70K to 40K, and the second-stage cold head H2 to be set from 20K to 4K. The cold heads of these stages cool an object to a desired temperature. Such a GM refrigerator has been disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2001-230459.
As a silicon single crystal manufacturing apparatus, a single crystal pulling device based on the Czochralski process (CZ process) has been used for fusing polycrystalline silicon to grow a single-crystal seed crystal. In the single crystal pulling device, silicon is fused in a crucible, generating thermal convection. This leads to deteriorated quality in generated single crystal in some cases. A method is known for restraining such convection by applying a magnetic field to the fused silicon so as to effect electromagnetic braking primarily to improve the quality of the generated single crystal. This method is called the magnetic Czochralski process (MCZ process). It has been known that a perpendicular magnetic field in a direction perpendicular to the liquid level of fused silicon, a horizontal magnetic field in a direction parallel to the liquid level of fused silicon, or a cusp magnetic field is applied to the fused silicon. Furthermore, a superconducting magnet apparatus having a GM refrigerator is used as a magnetic field applying device. This type of superconducting magnet apparatus normally includes multiple GM refrigerators.
Maintenance of the GM refrigerator R is required whenever the GM refrigerator R is used for a long time (about 10,000-hour operation) for cooling the superconducting magnet apparatus used with the single crystal pulling device. Normal maintenance includes replacement and inspection of parts constantly in motion. Performing the maintenance operation requires the motor drive M and the displacers D1, D2 connected thereto be pulled out and removed from the first and second-stage cooling cylinders C1, C2.
If, however, the displacers D1, D2 cooled to a low temperature should be pulled out of the first and second-stage cooling cylinders C1, C2 in the atmosphere without interrupting the operation of the superconducting magnet apparatus, then moisture in the air will instantly turn into a frozen film and adhere to the inner surfaces of the first and second-stage cooling cylinders C1, C2. The adhering frozen film can be temporarily removed by a dryer or the like. However, the first and second-stage cooling cylinders C1, C2 continue to be cooled in a cryogenic vacuum vessel. This causes the frozen film to be produced on the inner surfaces of the first and second-stage cooling cylinders C1, C2 again, thus preventing the maintenance operation from being performed.
Therefore, the operation of not only the single crystal pulling device but the superconducting magnet apparatus also had to be interrupted to increase the entire superconducting magnet apparatus to normal temperature (or room temperature) before starting a maintenance operation. For the superconducting magnet apparatus to be increased from 4K to the normal temperature after the operation of the superconducting magnet apparatus is stopped, it requires about 6 days to about 20 days although it depends on the sizes of coils thereof. Then, one day is spent to carry out maintenance on multiple GM refrigerators installed in the superconducting magnet apparatus. Thereafter, the operation of the superconducting magnet device is restarted to cool the coils, taking as the same number of days as that spent for increasing the temperature of the coils. The operation of the single crystal pulling device is not resumed until the coil temperature reaches 4K. Thus, the maintenance of the GM refrigerators takes a total of two weeks to almost one month and a half. The operation of the single crystal pulling device is suspended, resulting in a considerable operation loss.
As a possible solution to the aforementioned problem, the whole set of the GM refrigerators including the first and second-stage cooling cylinders C1, C2 may be replaced with another set that has already been maintained, rather than changing parts of the GM refrigerators requiring maintenance. FIG. 2 shows a proposed structure based on this design concept.
Referring to FIG. 2, a top plate 111 of a vacuum vessel 100 has a sleeve 2 with an opening in its top to provide isolation from a vacuum area in the vacuum vessel 100. The first and second-stage cooling cylinders C1 and C2, respectively, of the GM refrigerator R are inserted through the upper opening of the sleeve 2. This installs the GM refrigerator R such that the first and second-stage cooling cylinders C1, C2 are isolated from the vacuum area in the vacuum vessel 100.
The sleeve 2 has a first-stage sleeve 2a and a second-stage sleeve 2b. A lower end of the first-stage sleeve 2a has a first-stage cooling flange F1. The second-stage sleeve 2b has its upper end connected to the first-stage cooling flange F1, and has a second-stage cooling flange F2 provided at its lower end. The first-stage sleeve 2a has a flange F3 welded to the rim of its opening to air-tightly bolt it to the top plate 111 of the vacuum vessel 100. As previously mentioned, the flange 4 is also bolted to the top plate 111 of the vacuum vessel 100. The top plate portion of a heat shield vessel 106 is installed to the first-stage cooling flange F1 in such a manner to permit heat transmission. An object to be cooled, such as the superconducting magnet apparatus, is in contact with the second-stage cooling flange F2 so as to permit heat transmission.
Referring to FIG. 2, indium sheets 3a and 3b having a thickness of about 0.5 mm to about 1 mm are placed between the contact surfaces of the first-stage cold head H1 and the first-stage cooling flange F1 and between the contact surfaces of the second-stage cold head H2 and the second-stage cooling flange F2 in the GM refrigerator R to enhance thermal contact of these contact surfaces. The indium sheet 3a has a ring shape while the indium sheet 3b has a circular shape. Hereinafter, the contact surfaces will be referred to as the “thermal contact interfaces.”
In FIG. 2, the sleeve 2 is drawn using a single line, ignoring its wall thickness. A gap exists between the inner surface of the sleeve 2 and the outer surfaces of the first and second-stage cooling cylinders C1 and C2, the thermal contact interfaces being excluded. The thermal contact interfaces are orthogonal with respect to the direction in which the first and second-stage cooling cylinders C1 and C2 extend.
Using the sleeve 2 described above makes it possible to replace the whole set of the GM refrigerator without the need for increasing the superconducting magnet apparatus to the normal temperature. The aforementioned proposed structure, however, poses a problem when the new GM refrigerator is installed. More specifically, whenever the whole set of the GM refrigerator including the first and second-stage cooling cylinders C1 and C2 is pulled out from the sleeve 2 to replace it, the GM refrigerator assembly is unavoidably exposed. This causes air to get into the sleeve 2 of a cryogenic temperature. As a result, a frozen film formed by moisture or the like in the air adheres to the thermal contact interfaces of the cold heads and the sleeve 2 of the new GM refrigerator to be inserted. This leads to deteriorated thermal contact performance or heat transmitting performance.
The maintenance method using the above sleeve 2 presents the following disadvantages.
(1) A shielding unit is required to prevent air from getting into the sleeve when replacing the GM refrigerators.
(2) The GM refrigerators must be replaced in the shielding unit.
(3) The indium sheets between the thermal contact interfaces are rapidly cooled and hardened, causing them to lose their flexibility when they are in contact with the cold heads.
(4) When the GM refrigerators are replaced, if the first and second-stage cooling cylinders C1 and C2, respectively, of the GM refrigerators are inserted and fixed aslant, the contact area of the thermal contact interfaces is undesirably reduced.
(5) If a replacement failure happens, the maintenance method cannot be redone.
(6) The replacement work includes many steps, requiring skill to successfully perform it.